Stimulatory effect of circulated nicotine on rapidly adapting receptors in canine lungs

The objective of this study was to determine the effect of circulated nicotine on rapidly adapting receptors (RARs) in canine lungs. Afferent activities of RARs were studied with a single fiber recording technique in 10 anesthetized, open-chest and artificially ventilated dogs. Bolus injection of nicotine (10 micrograms/kg) into either right or left atrium did not activate the RARs in the first two breaths, but it consistently evoked a mild stimulatory effect beginning approximately 3-6 breaths after the injection, accompanied by a mild increase in tracheal pressure (Pt). There was no significant difference between the receptor responses to nicotine injected via these two different routes in either the peak discharge or the latency time. By contrast, nicotine delivered by cigarette smoke directly into the airways triggered an intense burst of activity upon the very first breath of smoke delivery. Taken together, these results suggest that the sensory endings of RARs are located near the surface of airway epithelium, more easily accessible from the luminal side of the airways. The delayed stimulation of RARs following the nicotine injection was probably activated by the bronchoconstrictive effect of nicotine.